For the past 150 years, humans have been performing an unprecedented experiment on Earth's climate. Human activities, mainly fossil fuel combustion, are increasing concentrations of greenhouse gases (GHGs) in the atmosphere. These gases are trapping infrared radiation emitted from the planet's surface and warming the Earth. Global average surface temperatures have risen about 0.7°C (1.4°F) since the early 20th century.

Earth's climate is a complex system that is constantly changing, but the planet is warmer today than it has been for thousands of years, and current atmospheric carbon dioxide (CO2) levels have not been equaled for millions of years. As we will see below, ancient climate records offer some clues about how a warming world may behave. They show that climate shifts may not be slow and steady; rather, temperatures may change by many degrees within a few decades, with drastic impacts on plant and animal life and natural systems. And if CO2 levels continue to rise at projected rates, history suggests that the world will become drastically hotter than it is today, possibly hot enough to melt much of Earth's existing ice cover. Figure 1 depicts projected surface temperature changes through 2060 as estimated by NASA's Global Climate Model.

Past climate changes were driven by many different types of naturally-occurring events, from variations in Earth's orbit to volcanic eruptions. Since the start of the industrial age, human activities have become a larger influence on Earth's climate than other natural factors. High CO2 levels (whether caused by natural phenomena or human activities) are a common factor between many past climate shifts and the warming we see today.

Many aspects of climate change, such as exactly how quickly and steadily it will progress, remain uncertain. However, there is strong scientific consensus that current trends in GHG emissions will cause substantial warming by the year 2100, and that this warming will have widespread impacts on human life and natural ecosystems. Many impacts have already been observed, including higher global average temperatures, rising sea levels (water expands as it warms), and changes in snow cover and growing seasons in many areas.

A significant level of warming is inevitable due to GHG emissions that have already been released, but we have options to limit the scope of future climate change—most importantly, by reducing fossil fuel consumption (for more details, see Unit 10, "Energy Challenges"). Other important steps to mitigate global warming include reducing the rate of global deforestation to preserve forest carbon sinks and finding ways to capture and sequester carbon dioxide emissions instead of releasing them to the atmosphere. (These responses are discussed in Unit 13, "Looking Forward: Our Global Experiment.")